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1. Chapter 9
Pathways that Harvest Chemical
Energy

2. Main Fuel Molecule: Glucose
Fuels: Molecules whose stored energy
can be released for use.
The most common fuel in organisms is
glucose. Other molecules are first
converted into glucose or other
intermediate compounds.

3. How is Glucose Used to Make Energy
Burning or metabolism of glucose:
C6 H12O6 + 6O2 → 6CO2 + 6 H 2O + free energy
Glucose metabolism pathway traps the
free energy in ATP:
ADP + Pi + free energy → ATP

4. How is Glucose Used to Make Energy
ΔG is the change in free energy
ΔG from complete combustion of
glucose
= –686 kcal/mol
Highly exergonic; drives endergonic
formation of many ATP molecules.

5. Three metabolic pathways are involved
in harvesting the energy of glucose:
Glycolysis: glucose is converted to
pyruvate
Cellular respiration: aerobic and
converts pyruvate into
H2O, CO2, and ATP
Fermentation: anaerobic and converts
pyruvate into lactic acid or
ethanol, CO2, and ATP

6. The Big Picture
If O2 is present (aerobic):
Glycolysis is followed by three
pathways of cellular
respiration:
• Pyruvate oxidation
• Citric acid cycle
• Electron transport chain
If O2 is not present (anaerobic):
Pyruvate from glycolysis is
metabolized by fermentation.

7. Redox Reactions
Redox reactions: One substance
transfers electrons to another
substance
Reduction: Gain of one or more electrons
by an atom, ion, or molecule
Oxidation: Loss of one or more electrons
Also occurs if hydrogen atoms are gained or
lost (H = H+ + e-)

8. Redox reactions

9. Redox Reactions and Glucose
In glucose combustion,
glucose is the reducing
agent, O2 is the oxidizing
agent.
Energy is transferred in a
redox reaction.
Energy in the reducing
agent (glucose) is
transferred to the reduced
product.

10. Electron Carriers
Examples: NAD,
FADH2, NADPH
Coenzyme NAD+ is a
key electron carrier
in redox reactions.
Two forms:
NAD+ (oxidized)
NADH (reduced)

11. Complete vs. Incomplete
Oxidation of Glucose
Incomplete Oxidation
Complete Oxidation

12. The five metabolic pathways occur
in different parts of the cell.

13. Glycolysis
Takes place in the cytosol
Converts glucose into pyruvate
Produces a small amount of energy
Generates no CO2

14. Glycolysis
Involves ten enzyme-catalyzed reactions.
Energy-investing reactions: Require ATP
Energy-harvesting reactions: Yield NADH and ATP.
Results (net per glucose):
2 molecules of pyruvate
2 molecules of ATP
2 molecules of NADH

15. “Energy Investing”
Steps of Glycolysis
So what is a kinase?

16. “Energy Harvesting”
Steps of Glycolysis

17. Substrate Level Phosphorylation
Enzyme-catalyzed transfer of a phosphate group
from a donor to ADP to form ATP is called
substrate-level phosphorylation.
Phosphorylation: addition of a phosphate group.

18. Anaerobic Conditions
Without O2, ATP can be produced by
glycolysis and fermentation.
Fermentation occurs in the cytosol, to
regenerate NAD+.
Pyruvate from glycolysis is reduced by
NADH + H+.

19. Lactic Acid Fermentation
• Occurs in
microorganisms,
some muscle cells
• Pyruvate is the final
electron acceptor
• Lactate is the
product and can
build up

20. Alcohol Fermentation
• Requires two
enzymes to
metabolize pyruvate
to ethanol
• Acetaldehyde is
reduced by NADH +
H+, producing NAD+
and glycolysis
continues

21. Summary of Anaerobic
Respiration
Cellular respiration yields more energy
than fermentation per glucose
molecule.
• Glycolysis plus fermentation = 2 ATP
• Glycolysis plus cellular respiration = 32
ATP
• So why do it?

22. Aerobic Respiration: Pyruvate Oxidation
Links glycolysis and the citric acid cycle;
occurs in the mitochondrial matrix
Pyruvate is oxidized to acetate and CO 2 is
released
NAD+ is reduced to NADH, capturing
energy
Some energy is stored by combining
acetate and Coenzyme A (CoA) to form
acetyl CoA

23. Pyruvate Oxidation
Per Glucose:
2 NADH
2CO2
2 Acetyl-CoA

24. Citric Acid Cycle
Inputs: acetyl CoA, water and electron
carriers NAD+, FAD, and GDP
Energy released is captured by ADP and
electron carriers NAD+, FAD, and GDP
Outputs: CO2, reduced electron carriers,
and ATP (really GTP)

25. Citric Acid Cycle
The citric acid cycle is in steady state:
The concentrations of the
intermediates don’t change.
The cycle continues when starting
materials are available:
• Acetyl CoA
• Reoxidized electron carriers

26. Citric Acid Cycle
Per Glucose:
6 NADH
2 FADH2
4 CO2
2 ATP

27. Complete Oxidation of
Glucose

28. Recycle Electron Carriers
The electron carriers that are reduced
during the citric acid cycle must be
reoxidized to take part in the cycle
again.
Fermentation—if no O2 is present
Oxidative phosphorylation—O2 is present

29. Oxidative Phosphorylation
Oxidative phosphorylation: ATP is
synthesized by reoxidation of electron
carriers in the presence of O2.
Two stages:
• Electron transport
• Chemiosmosis

30. Electron Transport Chain
(ETC)
Electrons from NADH and FADH2 pass
through the respiratory chain of
membrane-associated carriers.
Electron flow results in a proton
concentration (Membrane Potential)
gradient in mitochondria.

31. ETC
The respiratory chain is located in the
inner mitochondrial membrane
(cristae).
Energy is released as electrons are
passed between carriers.
Examples: protein complexes I, II, III, IV;
Cytochrome c, ubiquinone (Q)

32. ETC

33. Proton Motive Force
“Membrane Potential”
During electron transport protons are
also actively transported.
Protons accumulate in the
intermembrane space and create a
concentration gradient and charge
difference— potential energy!
This proton-motive force drives protons
back across the membrane.

34. Proton Motive Force
I, III, and IV Pump Protons

35. Chemiosmosis
Protons diffuse back into the
mitochondria through ATP synthase, a
channel protein.
Diffusion is coupled to ATP synthesis.
Oxidative Phosphorylation is one
example of Chemiosmosis

36. ATP Synthase
F0 subunit:
transmembrane
F1 subunit: projects
into the
mitochondrial
matrix, rotates to
expose active sites
for ATP synthesis

37. Summary of Aerobic
Respiration
Glycolysis ETC
4 ATP – 2ATP used =2 ATP NET NADH (TOTAL=10 x 2.5=25 ATP)
2 NADH FADH2 (2 x 1.5= 3 ATP)
(IF FERMENTATION use 2 NADH to Total ATP = 28
reduce pyruvate)
Pyruvate oxidation Total for Cell Respiration
2 NADH 4 ATP by substrate level
2 CO2 respiration
TCA Cycle 28 ATP by oxidative
2 ATP Phosphorylation
6 NADH = 32 ATP
4 CO2
2 FADH2